1. Field of the Invention
The present invention relates in general to rotary earth boring bits and, more particularly, to an improved thrust bearing surface between the bearing pin and the rotatable cone of the bit.
2. Brief Description of the Prior Art
A rotary rock bit generally comprises a main bit body adapted to be connected to a rotary drill string. The bit includes individual rotatable cone cutters mounted on bearing pins extending downwardly inwardly from an arm of the main bit body generally toward the center of rotation. Bearing systems are provided between each cone cutter and the associated bearing pin to reduce friction from rotation of the cutter as the bit and the cutters rotate during drilling. In order to obtain acceptable penetration rates with a rotary rock bit in some formations, it is necessary to apply heavy loads on the bit and to operate the bit at moderate speed.
The rotary rock bit operates under a highly abrasive and corrosive environment and is subjected to temperature extremes as the drilling operation is often conducted thousands of feet underground wherein elevated temperatures are encountered. The bit is cooled by continually flushing its exterior with a circulating drilling fluid which also carries away the drill cuttings. This fluid is generally corrosive and contains abrasive particles and would cause damage to bearing surfaces should it have access to them.
Thus, in view of these circumstances, it can be appreciated that a bearing system for a rotary rock bit must be constructed so that it will provide exceptional performance characteristics within a limited geometrical configuration. Since the entire drill string must be withdrawn to replace a bit should it fail, it is highly desirable to have the bearing systems of the bit operate for an extended period of time.
The development of journal bearing rotary rock bits has included the use of matched radial surfaces perpendicular to the axis of rotation between the journal pin and the cone cutter as thrust bearing surfaces. The radial surface on the journal pin usually contains a circular groove surrounding a pilot pin, extending axially from this surface. The groove is filled with a hardmetal which has a demonstrated ability to reduce friction and to withstand wear. To decrease friction and wear, the facing radial bearing surface in the cone cutter may be carburized. It has been common to provide such a surface with various indentations such as slots or drilled recesses containing a relatively soft alloy metal such as copper, silver, beryllium-copper, or aluminum-bronze to form a bearing pair with hardmetal against soft metal to reduce galling. In addition, it is common practice to grease each bearing surface with a "smear grease" prior to assembly of the cone cutter on the pin.
In U.S. Pat. No. 3,235,316 to J. R. Whanger, patented Feb. 15, 1966, a journal thrust bearing for a rock bit is shown with alternating surface areas of wear-resistant and anti-galling materials. The bearing system disclosed therein includes grooves in the bearing surface of the rotatable member filled with a soft metal having anti-galling characteristics to further reduce friction and wear between the two surfaces.